1. Field of the Invention
The present invention relates to a backlight unit used for a liquid crystal display and particularly relates to an edge light type backlight unit.
2. Description of the Related Art
Since a liquid crystal display (hereinafter, also referred to as an LCD) has a low power consumption and can be used as a space-saving image display device, the applications thereof have been expanding year by year. For example, the liquid crystal display is configured such that a backlight unit, a backlight-side polarizing plate, a liquid crystal panel, and a viewing side polarizing plate are provided in this order.
As a backlight unit, a direct backlight unit which includes a light source disposed below an emission surface thereof or an edge light type backlight unit (also referred to as a side light type backlight unit in some cases) which includes a light source disposed on a side of an emission surface thereof has been known.
A backlight unit has a great impact on the performance of an LCD such as the luminance or visibility of an image. Accordingly, various suggestions for improving the luminance of light (backlight) emitted by a backlight unit have been made.
For example, it has been suggested that a reflective polarizing plate is used as a film for improving the luminance A reflective polarizing plate transmits predetermined polarized light and reflects polarized light other than the predetermined polarized light. In a case where a reflective polarizing plate is used, the reflective polarizing plate transmits only linearly polarized light responding to a backlight-side polarizing plate, allows the transmitted light to be incident on the backlight-side polarizing plate, reflects polarized light other than the linearly polarized light so that retroreflection is repeated in a backlight unit, and allows the reflected light to be incident on the reflective polarizing plate again so that the light can be reused.
For example, JP1998-3079A (JP-H10-3079A) describes a backlight unit that uses a reflective polarizing plate reflecting circular polarization in a predetermined direction and transmitting other circular polarization due to a cholesteric liquid crystal. In this backlight unit, the circular polarization transmitted through the reflective polarizing plate is allowed to be incident on a λ/4 plate and then to be incident on a backlight-side polarizing plate as linearly polarized light in a predetermined direction.
While the circular polarization reflected by the reflective polarizing plate that separates circular polarization is reflected in the backlight, a change in polarizability, for example, change of right circular polarization into left circular polarization, is shown. In a case where this circular polarization is incident on a reflective polarizing plate that uses a cholesteric liquid crystal layer again, the circular polarization is transmitted through the reflective polarizing plate. In other words, by using the reflective polarizing plate that reflects circular polarization, almost all light can be efficiently used through the extremely small number (one to several times) of times of reflection.
JP2015-518259A discloses a configuration of an edge light type backlight unit which uses a light guide box having a cavity therein without including a light guide plate as a configuration of a backlight unit that is capable of uniformizing light intensity distribution in the plane of emitted light and has attempted weight reduction and cost reduction. The light guide box guides light between a reflection sheet provided on one surface thereof and a light leakage sheet facing the reflection sheet and having a plurality of light leakage holes provided in a light emission opening of the light guide box. The surface of the light leakage sheet on the light guide box side is a reflection surface and the surface on the opposite side is an irregular reflection surface.